The invention relates to a particle-optic illuminating and imaging system, particularly for a transmission electron microscope, with a condenser-objective single field lens.
Modern electron microscopes are mostly equipped with a so-called condenser-objective single field lens (hereinbelow also xe2x80x9cKOE lensxe2x80x9d) according to Riecke and Ruska, as known, for example, from [German Patentschrift] DE-PS 875555. Condenser-objective single field lenses differ from conventional objectives in that the specimen is arranged in the middle of the pole shoe gap. By the arrangement of the specimen in the magnetic field maximum, reduced image error coefficients result in contrast to conventional objectives. Furthermore, the realization of eucentric goniometers and bar locks is substantially simplified, and in contrast to conventional objectives a greater constructional space is available for specimen tilting and for the arrangement of detectors, for example for the detection of X-rays emitted from the specimen or for the detection of back-scattered electrons. Furthermore, condenser-objective single field lenses offer the possibility of realizing very small electron probes for point analysis and for scanning operation (so-called STEM operation) with the condenser-objective single field lens.
In connection with the illuminating devices for condenser-objective single field lenses, it has heretofore been taken as a starting point that in TEM operation an imaging of the crossover of the particle source in the rear focal plane of the condenser-objective single field lens is imperatively required in order to prevent a troublesome oblique specimen illumination in off-axis regions. A corresponding illumination system which manages with only two condenser lenses is known from U.S. Pat. No. 3,560,781. In the said system, in order to make possible the different illumination apertures which are required at different magnifications, the pole shoes of the condenser lens immediately following the particle source are interchangeable, in order to attain different imaging scales for the imaging of the crossover of the particle source by means of the first condenser lens. The second condenser lens is always operated with the same fixed excitation, so that the crossover image produced by the first condenser lens is always imaged in the illumination-side focal plane of the condenser-objective single field lens. The interchanging of the pole shoes proposed here is however impractical for a serial equipment in practice, because of the required precision.
Variation of the excitation of both condenser lenses for setting different illumination apertures has already been proposed in U.S. Pat. No. 3,560,781, as an alternative to interchanging the pole shoes. However, the size of the field illuminated on the specimen is exclusively determined by the size of the condenser diaphragm in such an operation of the illuminating system. A change of the field size is therefore only possible by means of a mechanical change of the condenser diaphragm.
A so-called twin-objective lens is known from DE-C 28 22 242, in which a further lens gap is provided in the ferromagnetic circuit. The condenser diaphragm is imaged in the condenser-side focal plane of the condenser-objective single field lens with the aid of this constantly excited auxiliary lens. In this system, the illumination aperture can be varied by variation of the excitation of the second condenser lens. It is however disadvantageous that in STEM operation the field of the auxiliary lens has to be compensated by an additional coil.
A further illumination system is known from U.S. Pat. No. 4,633,085; its function largely corresponds to the function of the illumination system of DE-C 28 22 242. In U.S. Pat. No. 4,633,085, the auxiliary lens is realized solely by means of a snorkel lens arranged immediately before the objective.
Although the illumination systems of DE-A 28 22 242 and U.S. Pat. No. 4,633,085 respectively have three condenser lenses, these systems do not permit independent setting both of the illuminated field and also of the illumination aperture.
An illumination system is known from U.S. Pat. No. 5,013,913, in which the illumination aperture and the illuminated field can be set independently of each other. This system contains three condenser lenses, which image the crossover of the particle source with variable magnification, always in the source-side focal plane of the condenser-objective single field lens. The size of the crossover image then defines the illumination aperture. The size of the illuminating field is determined by the size of the condenser diaphragm, which can be varied by electron-optical selection of a suitable diaphragm from a multi-hole diaphragm. This illumination system always makes possible an optimum illumination of the specimen; this advantage is however gained at the expense of a relatively complex construction.
The present invention has as its object to provide a simple illuminating system for a condenser-objective single field lens, which makes possible a continuous variation of the illuminating field size and in which an increase of the illumination aperture accompanies a decrease of the illumination field.
This object is attained according to the invention by the particle-optic illuminating and imaging system with the features of claim 1. Advantageous developments will become apparent from the features of the dependent claims.
The particle-optic illuminating and imaging system according to the invention has a condenser-objective single field lens, two condenser lenses arranged between a particle source and the condenser-objective-single field lens, and a multi-lens imaging system arranged after the condenser-objective single field lens. In TEM operation, exclusively the excitation of the condenser lens adjacent to the particle source is varied. The excitation of the condenser lens adjacent to the condenser-objective single field lens is independent of the set size of the illuminating field in TEM operation.
TEM operation is here understood to be such operating conditions that either the diameter of the illuminated field in the specimen plane is xe2x89xa70.5 xcexcm, or the illumination aperture is xe2x89xa65 mrad.
It was recognized according to the invention that the required apertures and sizes of the illuminated field in TEM operation can be realized with an illuminating system having only two condenser lenses, even with a condenser objective single field lens, solely by variation of the excitation of the source-side condenser lens. The illuminating and imaging system according to the invention therefore preferably has exactly two condenser lenses.
In setting a maximum illuminating field diameter, the source-side condenser lens has to be relatively strongly excited, so that it images the crossover of the particle source with a reduction of about 20 to 50 times. The excitation of the condenser lens on the objective side is then to be set so that this second condenser lens images the crossover produced by the first on a scale of about 1:1 in the condenser-side focal plane of the condenser-objective single field lens. At the maximum illuminating field diameter, there thereby results an axially-parallel, coherent illumination of the regions remote from the axis, so that oblique illumination of the regions remote from the axis does not arise in this setting.
For setting smaller illuminating field diameters, the excitation of the condenser lens on the particle source side is suitably reduced, so that the image of the crossover produced by the source-side condenser lens shifts along the optical axis toward the objective-side condenser lens. The smallest illuminating field is then attained when both condenser lenses in common image the crossover of the particle source in the plane conjugate to the specimen plane with respect to the condenser-objective single field lens. The condenser-objective single field lens then images this intermediate crossover onto the specimen. At smaller illuminating field diameters, there admittedly results an oblique illumination of the off-axis regions of the specimen. Since, however, the oblique illumination increases with decreasing illumination field diameter, this does not have very troublesome effects.
So that the range of field diameters usually required in TEM operation can be realized with the two-stage condenser, the fixed imaging scale of the objective-side condenser lens had to be greater than 1:3, preferably between 1:1 and 1:3 (the maximum reduction of the crossover image about ⅓ by the second condenser lens at maximum illuminating field diameter).
In order to limit the illumination aperture at small illuminating field diameters, a front field diaphragm is to be provided in the source-side focal plane of the condenser-objective single field lens. This front field diaphragm has to be interchangeable so that large illumination apertures are attainable with a spot illumination.
With the beam paths described above, the smallest attainable illumination field diameters are in the region of 1 xcexcm. For setting smaller illuminating field diameters, both condenser lenses (in the so-called spot or STEM mode) are operated as a zoom lens, with both condenser lenses always imaging the crossover of the particle source into the condenser-side input image plane of the condenser-objective single field lens. The condenser-objective single field lens then images the intermediate image of the crossover of the particle source, once again reduced, onto the specimen. The spot size can then be varied by different excitation of the two condenser lenses.
In the illuminating and imaging system according to the invention, a single excitation value can consequently be set for the objective-side condenser lens in TEM operation.